The present invention relates to a modified natural rubber obtained by using a deproteinized natural rubber which is substantially free from any protein, a process for improving the graft ratio and the graft efficiency of natural rubber, and a process for improving the epoxidation ratio of natural rubber without gel formation.
Natural rubber has been widely employed in industrial articles such as automobile tires, belts and adhesives and domestic articles such as gloves.
In addition to its excellent mechanical properties as vulcanized rubber, natural rubber is much superior in the raw rubber strength (green strength) to synthetic rubbers. Accordingly, natural rubber is excellent in processing characteristics in kneading, sheeting and various molding procedures. In the form of a latex, natural rubber has a high gel strength at solidification and thus can be easily a formulated into a film, which makes natural rubber applicable to various products including condoms, surgical gloves and diverse catheters.
However, natural rubber cannot compete successfully with synthetic rubbers which have specific characteristics, for example, butyl rubber having a high gas permeability or nitrile rubber being excellent in oil resistance.
Therefore, attempts have been made to modify natural rubber so as to impart the characteristics of other rubbers thereto while maintaining the excellent mechanical properties and film-forming properties of the natural rubber. Examples of known modification methods include graft copolymerization with an organic compound having an unsaturated bond and epoxidation. In the graft copolymerization with an organic compound having an unsaturated bond, there have been employed methyl methacrylate, styrene, acrylonitrile, etc. as a monomer. Among these there has been marketed a graft copolymerization product of natural rubber with methyl methacrylate as xe2x80x9cMG Latexxe2x80x9d.
In general, natural rubber is modified by these treatments in the state of a latex stabilized with a surfactant by taking the cost and the easiness in handling into consideration, though a solid rubber or a rubber solution is subjected to these treatments in some cases.
However, a natural rubber latex just collected from a rubber tree usually contains about 5% of non-rubber components including proteins. Also, a commercially available concentrated latex contains about 3% of non-rubber components. Thus, there arises a problem that these non-rubber components, in particular, proteins inhibit the modification of natural rubber and, in the case of graft copolymerization, the graft ratio and the graft efficiency are lowered, which makes it impossible to achieve a high modifying effect.
On the other hand, it has been recently reported in United States that medical instruments with the use of natural rubber such as surgical gloves, various catheters and anesthetic masks cause dyspnoea or anaphylactoid symptoms (for example, angioedema, urticaria, collapse, cyanosis). It has been also reported that female patients with allergic anamnesis suffered from hand ache, urticaria and angioedema around eyes due to the use of household natural rubber gloves.
It is assumed that these symptoms are caused by the proteins contained in natural rubber. It has been therefore required to eliminate the proteins from natural rubber products. These problems accompany not only natural rubber products but also modified natural rubbers obtained by modifying the natural rubber.
In addition, natural rubber has another disadvantage that the material properties thereof vary depending on the production area and production time, which is a problem characteristic of natural products. Therefore, the elimination of the non-rubber components causing this problem makes the vulcanizing characteristics stable. Thus natural rubber becomes a material rubber having stable qualities comparable to synthetic rubbers. In addition, the mechanical accuracies of modified natural rubber products can be elevated.
There have been marketed in practice crepe H, crepe G and crepe CD as a deproteinized natural rubber.
The protein content of natural rubber is generally expressed in an amount corresponding to 6.3 times as much as its nitrogen content which is determined by the Kjeldahl method. According to the present inventors"" examination, the nitrogen content of a fresh natural rubber latex (field latex) ranges from about 0.5 to 0.9% by weight, while those of a marketed refined latex and raw rubber (smoked sheet rubber) are about. 0.3% by weight or above. Although the protein contents of the conventional deproteinized natural rubbers are remarkably lower, the nitrogen content of crepe CD, which has the lowest protein content, is still about 0.11% by weight. Thus, they are not completely deproteinized yet and, therefore, cannot elevate the modification efficiency. Also they are unsatisfactory as a material for preventing allergy.
An object of the present invention is to provide a modified natural rubber which has been modified at a high efficiency and a process for producing the same.
Another object of the present invention is to provide a modified natural rubber having no allergy causing potential.
Still another object of the present invention is to provide a process for improving the graft ratio and the graft efficiency of natural rubber and a process for improving the epoxidation ratio of natural rubber.
The modified natural rubber of the present invention for achieving the above-mentioned object is one obtained by a modification of a deproteinized natural rubber having a nitrogen content of less than 0.10% by weight.
The process for improving the graft ratio and the graft efficiency of a graft-copolymerized natural rubber comprises deproteinizing natural rubber and then graft-copolymerizing the natural rubber.
The process for improving the epoxidation ratio of an epoxidized natural rubber comprises deproteinizing natural rubber and then epoxidizing the natural rubber.